The present invention relates generally to a method and apparatus for supporting a structure such as a building, bridge, or power plant such that it is isolated from seismic vibratory ground motion. More particularly, the present invention relates to a method and apparatus for supporting a structure by an isolation system which will not allow large dynamic loads to be transmitted to the supported structure due to seismic motions which have damaging energy at frequencies at or near the natural frequency of the structure and the overall structural systems.
Two methods are used to prevent damage to buildings and other structures due to vibratory ground motion of seismic events. One method, the current conventional approach, is to embed the base of the structure onto firm soil and construct the structure to withstand seismic motion by providing adequate strength, rigidity and ductility. Use of this conventional method may incur significant costs of construction. In addition, it permits the seismic motion to be passed upward through the structure with resulting amplification of seismic accelerations and forces, thus requiring further precautions to prevent possible injury or death to occupants and costly damage to contents. The second method for preventing structural damage due to seismic events commonly known as seismic isolation, diminishes the seismic forces passed on throughout the structure, i.e. it decouples the structure from the earthquake ground shaking by supporting the base of the structure on a system of isolator bearings, which are, in turn, supported by a lower foundation mat, which is embedded upon the soil or rock.
The seismic isolation method of protecting a structure and its contents from seismic motion is based on the response of structures to vibrating motion at the base of the structure. If the fundamental natural frequency of the support and structure is sufficiently below the dominant or high energy content frequencies of the seismic motion, the structure will be subjected to greatly reduced seismic loading as compared to the more conventional method. Historical records of many damaging earthquakes in the world show that the seismic motion is observed to have most of the damaging potential energy at frequencies between 1 and 10 Hz. At the present time, seismic isolation systems are typically designed to create a support and structural system which has a natural frequency of less than 1.0 Hz, in some cases as low as 0.1 Hz.
Structures and equipment and others contents within the structure are generally more susceptible to damage from seismic motion in the horizontal plane than in the vertical direction. A common practice in seismic isolation design, therefore, is to isolate the structure from ground motion only in the horizontal plane. This isolation is achieved by supporting the structure by isolator bearings which, with the structure, form a dynamic system that has a horizontal natural frequency much lower than the non-isolated structure. The vertical natural frequency is usually designed to be very high, and is greater than the dominant frequencies of the vertical seismic ground motion. The structure will consequently not be significantly excited by the expected seismic event; it is noted, however, that certain portions of the structures such as beams and slabs, and certain equipment or other contents within the structure may have lower natural frequencies, and may be subjected to the same amplified considerations and loads experienced in the conventional design method.
Structures and in particular, components and systems contained within the structures, that are inherently susceptible to damage due to excitation by seismic motion are most efficiently protected by the isolation method. An example of this is a Nuclear Liquid Metal Reactor (LMR) vessel which is a thin walled structure having low natural frequencies and is therefore susceptible to seismic damage. The reactor vessel is a critical component of the reactor which must be reliably protected from earthquakes. Isolation of the reactor as a means of protecting it from damage must be achieved with a high degree of a reliability to assure safety of the LMR. Other highly critical facility examples include emergency facilities such as hospitals, in which many items of equipment as well as staff and patients are mobile and very vulnerable to horizontal seismic forces.
The frequency content of seismic motion at a given site is dependent on a variety of variables such as the geology of the site and consequently is best described as random. As a consequence, seismic motion that has significant energy at or near the isolation frequency is a possibility.
Therefore, in view of the above, an object of the present invention is to provide a method and apparatus to isolate a structure from seismic motion that will not allow large seismic loads to be transmitted to the structure. Another object of the present invention is to increase the assurance that an isolation system will not allow large seismic loads to be passed to a structure beyond that provided by isolation systems that do not provide for significant alteration of the natural frequency of the structure and isolation system. Additional objects, advantages and novel features of the invention will be set forth in part in the description that follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.